All About Sunspots

sunspot032901In two earlier issues, you learned how to observe the sun (and more importantly, how not to observe the sun).

But why bother observing the sun?  What is there to see on our home star?

The answer is… not much.  At least right now, though this will likely change very soon.  As we’ll explain in coming issues, the sun is strangely quiet this year, which has astronomers deeply puzzled.  But most expect the sun will get back to its old self soon enough and generate a number of dazzling sights for observers on Earth.

One of the easiest sights to see in a small telescope with a solar filter are sunspots.  These are small dark markings in the sun’s photosphere, which is the layer of the sun we can see in visible light.

Sunspots are like the hurricanes of the photosphere, large storms not of rain and wind, but of intense magnetic activity caused by twisting tubes of magnetic flux deep inside the sun.  These tubes wind through the innards of the sun and pop out into the surface from time to time.  Because they’re magnetic in nature, sunspots always occur in pairs with opposite magnetic polarity (just like magnets here on Earth).

When a magnetic tube pops through the surface, it interrupts heat flow from deeper layers in the sun and drops the local temperature to about 4,500 K, cooler than the 5,800 K temperature of the surrounding gas.  That’s what makes a sunspots dark: it’s simply cooler than the rest of the photosphere.  The coolest and darkest area is the umbra; the slightly hotter and brighter area surrounding the umbra is the penumbra.


Close-up of a sunspot

When the sunspots return, you’ll easily see them with a properly-equipped small telescope, even at low magnification.  But some larger spots are visible without a telescope (with a proper filter).  In fact, Chinese astronomers were the first to see these spots in 28 B.C., probably when the sun was low on the horizon and atmospheric dust dimmed the sun enough to see without a filter.  Legend has it that a large spot was also seen in 813 A.D. upon the death of Charlemagne.

Galileo, Thomas Heriot, and David Fabricius first observed sunspots with a telescope in 1610.  Their observations revealed an astonishing fact for the time… the sun rotated.  They determined this by tracking sunspots across the face of the sun from day to day.  This discovery was another blow to Aristotle’s view that heavenly objects were perfect and unchanging, and a boost to Copernicus’s view that the Earth was not the center of the universe.

You can use sunspots to see the sun’s rotation for yourself.  The sun rotates once every 24 days or so at the equator, and a little slower towards the poles (remember… the sun is not solid).  Spots move from left to right across the surface as seen from the northern hemisphere (opposite in the south).  Most spots don’t make it all the way around the sun, alas, since they have an average lifespan of just 2 weeks.

Other stars have spots, as well.  Called starspots, these patches can’t be seen directly, but are inferred by measurements of the effects of magnetic fields and rotation upon the narrow emission lines of atoms on the star’s surface.